1. Field of the Invention
The present invention relates to a data communication network using a cable television (CATV) network, and in particular, to a data communication network using coaxial lines between a head end and subscriber terminals.
2. Description of the Related Art
In general, a data communication network using a CATV network suffers much upstream ingress noise, which affects communications between a head end and terminals such as a cable MODEM (Modulator-Demodulator).
FIG. 1 is a block diagram of a general data communication network using a CATV network. In the data communication network, cable MODEMs 106 provided to subscribers' houses are connected to a head end 100 via a hybrid fiber coax (HFC) network. Here, a downstream path refers to a data transfer path from the head end 100 to the respective cable MODEMs 106, whereas an upstream path refers to a reverse data transfer path. A downstream band is in the range of 54 MHz to 750 MHz, and an upstream band is in the range of 5 MHz to 42 MHz. Between the head end 100 and the cable MODEMs 106 is connected a bidirectional amplifier 102. A bidirectional coupler 104 is connected between the bidirectional amplifier 102 and each cable MODEM 106. A coaxial line or an optical fiber is used to connect the head end 100 with the bidirectional amplifier 102, and a coaxial line is used to connect the bidirectional amplifier 102 with the bidirectional couplers 104. The bidirectional couplers 104 are also connected to the cable MODEMs 106 by a coaxial line. That is, the head end 100 is connected to the bidirectional couplers 104 via the HFC network. When connecting a coaxial line with each cable MODEM 106, an F-type connector is used.
FIG. 2 is a block diagram illustrating connection between such a cable MODEM and a coaxial line. A cable MODEM 200 corresponds to each of the cable MODEMs 106 shown in FIG. 1. Generally, an F-type male connector 202 is on the side of the cable MODEM 106, while an F-type female connector 204 is on the side of the coaxial line. In most cases, connection of the cable MODEM 200 to a coaxial line 206 with the F-type connectors is just made mechanically, but not verified afterwards. That is, after the cable MODEM 200 is connected to the coaxial line 206 with the F-type connectors, it is not checked whether they are tightly connected by repeatedly turning the F-type female connector 204. Therefore, user manipulation may loosen the connection between the cable MODEM 200 and the coaxial line 206.
Down stream data experiences no ingress noise in a general CATV network. On the contrary, noise in the band of 5 MHz to 42 MHz, generated from various electrical appliances including a hair dryer, a washing machine, a vacuum cleaner, and the like, may be introduced into upstream data through the loosely connected cable MODEM 200 and coaxial line 206 because each cable MODEM 106 transfers data to the head end 100 with a different frequency spectrum. This ingress noise is loaded on a radio frequency (RF) signal output from the cable MODEM 106 and amplified in the bidirectional amplifier 102, and reaches the head end 100. Ingress noise can be simultaneously introduced from different cable MODEMs 106 and amplified, resulting in intermodulation of different noise signals. Hence, a signal-to-noise (S/N) ratio remarkably drops in the head end 100, thereby adversely influencing communications between the head end 100 and the cable MODEMs 106.
To block such ingress noise, it should be determined from which cable MODEM the ingress noise was produced. However, it is impossible in the prior art to track back the origin of an ingress noise-including RF signal, let alone block it. Therefore, the RF signal including ingress noise is transferred to the head end 100 and impedes data communications through the HFC network.